1. Field of the Invention
The present invention relates to electronic circuits, and more particularly a voltage-controlled oscillator.
2. Description of the Related Art
Wireless communication needs worldwide have increased dramatically in the last few years, particularly in the amount of devices being used and the demand for higher data rates. The amount of available radiofrequency (RF) spectrum is limited, and the result has been an incredible increase in the licensing costs for specific frequency bands. The limited frequency spectrum has prompted telecommunication equipment manufacturers to either decrease the amount of bandwidth required for a specific data rate or to increase the data rate while staying within a specified bandwidth.
The highest data rate that a RF communication system can handle is generally determined by the type of modulation scheme used by the communication system. One of the most spectrally-efficient modulation schemes currently available is Quadrature Amplitude Modulation (QAM). There are many parameters in a RF system that can limit the order of QAM used. One of the main parameters is xe2x80x98phase noise.xe2x80x99 A high proportion of phase noise is generated by local oscillators (LOs) used in frequency conversion in a RF system. Many local oscillators use a voltage-controlled oscillator (VCO) within a phase locked loop (PLL) to generate a highly stable local oscillation with low phase noise.
A low phase noise voltage-controlled oscillator (VCO) is provided in accordance with the present invention. In one embodiment, the VCO comprises a negative resistance generator and a resonator structure that reduces VCO phase noise. The VCO uses the reflection properties of the resonator.
An advantage of one embodiment of the VCO is its relatively low cost of manufacture compared to other VCOs. Another advantage of one embodiment of the VCO is its high performance capability.
Another advantage of one embodiment of the VCO is its lower phase noise compared to conventional microstrip resonators. In one embodiment, low phase noise performance is achieved by tuning the outside fingers of an interdigital filter resonator in the VCO and configuring suitable physical dimensions of the resonator.
One aspect of the invention relates to a voltage-controlled oscillator comprising a resonator and a negative resistance generator. The resonator comprises a three-finger interdigital filter and a plurality of varactors. A first varactor is coupled to a first finger of the interdigital filter and a ground terminal. A second varactor is coupled to a third finger of the interdigital filter and a ground terminal. A second finger of the interdigital filter is coupled to a ground terminal. The first and second varactors are configured to alter a resonant frequency of the interdigital filter to a desired frequency in response to a tuning voltage applied to the resonator.
The negative resistance generator is coupled to the resonator. The negative resistance generator is configured to receive a first signal with a particular frequency from the resonator. The negative resistance generator is configured to output a second signal with a substantially similar frequency and a higher amplitude compared to the first signal.
Another aspect of the invention relates to a method of generating an oscillating signal at a desired frequency. The method comprises applying a tuning voltage to a resonator. The resonator comprises an interdigital filter and a plurality of varactors. The interdigital filter comprises a first finger, a second finger and a third finger. The first finger is coupled to a first varactor, and the third finger is coupled to a second varactor. The method further comprises using the tuning voltage to alter a resonant frequency of the interdigital filter to generate a signal with a desired oscillation frequency.